1. Field of the Invention
The present invention relates to an inkjet recording type image forming apparatus and a method for controlling the inkjet recording type image forming apparatus.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile, and a copier, an inkjet recording type image forming apparatus using a recording head that discharges ink droplets has been known. Such a liquid discharge recording type image forming apparatus performs image formation by discharging ink droplets from the recording head onto a conveyed recording medium (hereinafter referred to simply as “sheet” in some cases). In such liquid discharge recording type image forming apparatuses, there are a serial type image forming apparatus that forms an image by discharging liquid droplets while a recording head is moving in a main scanning direction, and a line type image forming apparatus using a line type head, which forms an image by discharging liquid droplets in a state where a recording head is not moving.
In such an image forming apparatus (hereinafter also referred to simply as “inkjet recording apparatus”), for example, there has been known an apparatus in which a head tank (sub-tank) for supplying ink to a recording head is mounted on a carriage on which the recording head is mounted, an ink cartridge (main tank) is detachably mounted on an apparatus body side, and ink is supplied from the ink cartridge on the apparatus body side to the head tank.
A pressure in the head tank used in such an apparatus is required to be held at a normal pressure for preventing ink leakage and inclusion (suction) of air from inside a nozzle. Therefore, the following controlling technique has been known. While the pressure in the head tank is held at a normal pressure by using a pressure sensing function of a unit for sensing a pressure in the head tank, ink is supplied and a negative pressure is formed.
However, by the conventional methods of ink supplying and ink discharging at the time of negative pressure formation, there have been the following problems. When the pressure sensing function in the head tank does not function due to a breakdown of a pressure sensing lever and the like, a machine operation is not stopped even when a pressure in the target head tank is at a target pressure value, and the ink continues to be supplied and discharged. If the ink is supplied even after the target negative pressure value is achieved, ink is excessively supplied and leaks out from the head tank. Further, when the ink continues to be discharged even after the target negative pressure value is achieved, there has been a problem in that the pressure in the head tank becomes an excessive negative pressure, whereby air is suctioned from inside the nozzle and a defect of discharge of the ink occurs.
Factors that prevent the pressure sensing function from functioning are: 1. defect of an atmospheric opening mechanism of the head tank; 2. adhesion of a filler; 3. abnormality of a full state sensor; and the like.
The “1. defect of an atmospheric opening mechanism of the head tank” is a defect where the atmospheric opening mechanism does not function even when an atmospheric opening valve is pressed. For example, there are (1-1) an atmospheric opening mechanism defect caused by liquid intruding into an atmospheric opening part, (1-2) an atmospheric opening mechanism defect caused by pressing the atmospheric opening valve in a state where a carriage is misaligned, and the like.
As for the defect (1-1) described above, there are cases where the atmospheric opening mechanism does not easily function due to an intrusion of liquid in the atmospheric opening part. In such a situation, there is a case where it takes time to perform atmospheric opening even by pressing the atmospheric opening valve, whereby a filler keeps a slightly closed state.
As for the defect (1-2) described above, there is a case where the carriage is misaligned due to skipping of reading of a main scanning part encoder. Even by pressing the atmospheric opening valve in the case of the misaligned carriage, the atmospheric opening does not function, and the filler is in a slightly closed state.
The “2. adhesion of a filler” is a defect where the filler is adhered to something and does not move, so the pressure sensing function does not function. For example, there are (2-1) an adhesion to a partition between heads, (2-2) an adhesion to the head tank, and the like.
The defect (2-1) is as follows. Plural heads in the carriage are separated by partitions per head. When the filler is opened at the time of atmospheric opening and the like, the filler sometimes contacts the partition between the heads. A contact part of the filler may adhere by “adhesion by electrostatic force” and “adhesion by surface tension or an adhesion force due to an attached liquid, solid, or the like”.
The defect (2-2) is as follows. When ink is consumed, the filler is closed and may contact the head tank. A contact part of the filler may adhere by “adhesion by electrostatic force” and “adhesion by a surface tension or an adhesion force due to an attached liquid, solid, or the like”. Even when ink is supplied in this state, the filler does not move. In addition, the filler is not moved even by pressing the atmospheric opening.
The “3. abnormality of a full state sensor” is a defect where there is an abnormality in the full state sensor, so that the filler cannot be accurately sensed and the pressure sensing function does not function. There are some cases where the sensor senses other matter than the filler by mistake, such as “paper dust”, “mist”, and “ink droplet”. As a result, the filler does not function.
For example, Japanese Patent Application Publication No. 2008-143073 (Patent Document 1) discloses the following mechanism against the ink leakage and inclusion (suction) of air from inside the nozzle. In this mechanism, a pressure sensing function using a pressure sensing lever is used to supply ink to achieve a target negative pressure value, for preventing an ink leakage caused by an excessive ink supply.
Japanese Patent Application Publication No. 2005-125667 (Patent Document 2) suggests a liquid discharge apparatus including ink supply piping for supplying ink from an ink tank to an ink head, and ink circulation piping for returning the ink from the ink head to the ink tank. The ink is circulated between the ink head and ink tank for preventing the ink leakage from a nozzle.
Japanese Patent Application Publication No. 2003-341028 (Patent Document 3) suggests an inkjet printer having an intermediate ink tank between an ink cartridge and a recording head, and a pressure sensor mounted in a place apart from the intermediate ink tank. A negative pressure state inside the recording head is sensed by the pressure sensor and the intermediate ink tank is moved up and down as required to maintain a desired negative pressure.
Japanese Patent No. 2898746 (Patent Document 4) suggests an inkjet pen in which ink is supplied in a sealed ink tank, a small hole having one end opened to the atmosphere is provided in this ink tank, and air is supplied through the small hole into the ink tank when the ink in the ink tank is consumed, whereby pressure in the ink tank is kept at a desired negative pressure.    Patent Document 1: Japanese Patent Application Publication No. 2008-143073    Patent Document 2: Japanese Patent Application Publication No. 2005-125667    Patent Document 3: Japanese Patent Application Publication No. 2003-341028    Patent Document 4: Japanese Patent No. 2898746
In the techniques described in these documents, however, the problem of “ink leakage” and problem that “pressure in the head tank becomes an excessively negative pressure” cannot be solved in a state where the pressure sensing function of the pressure sensing lever for sensing a pressure in the head tank does not function due to a breakdown and the like.